Hi Leo:
First off, congrats on being on a bike at all, at any speed! If my math is right you have 76 years on those knees and the soft stuff inside the chest. And if you can still do 20 mph, and read the speedo while doing it, better still! I hit 60 myself recently but can still give my Colnago C-50 a workout. I meet guys in the very steep Oakand/Berkeley hills all the time in their 60s and even 70s. Did a time trial up Mt. Diablo a few years back (3700 feet up in seven miles); one of the earlier finishers was 80! If your doc says you are fit to ride, keep at it. Overall fitness increases one's appreciation of life and gets the feel-good endorphins flowing, too.
And by the way the top teams in the Tour de France are resolutely and proven drug-free. Catch tomorrow's Tour stage up L'Alpe d'Huez!